Japanese Patent Publication No. 2001-123809 describes a lift and operation angle varying mechanism that is one example of a variable valve mechanism for internal combustion engines of automobiles and the like, that can continuously modify intake or exhaust valve characteristics, i.e. valve lift amount and operation angle, to attain improvements in fuel economy at low speeds and output at high speeds. In this mechanism, a rotating drive shaft connected to a crankshaft is linked by a plurality of links to oscillating cams that are valve cams, which lift intake and exhaust valves so that the lift characteristics are changed by changing the position of the rocker arm, which is one of these links. In this kind of variable valve mechanism, variation in the amount of valve lift unavoidably occurs due to dimensional accuracy among multiple link connection points. In particular, when the amount of valve lift (operation angle) is in the ultra-small lift region of 1 mm or less for example, air intake volume will fluctuate comparatively widely due to small variations in the valve lift between cylinders, so there is a possibility that engine operability and stability could be hindered. Therefore, typical prior art mechanisms allow for the adjustment of the amount of the valve lift of intake and exhaust valves by providing a variable valve mechanism comprised of specific links that can be selected and replaced from among a plurality of links of differing dimensions and grades.
However, in this kind of method, the operation can sometimes be extremely difficult, depending on the position and direction of the portion of the adjustment tool that mates/inserts into the bolt. In general, there is limited space inside cylinder heads where many parts, such as valve mechanism links, are located, and therefore, if for example, the portion of the adjustment tool that mates/inserts into the bolt is located inside a cylinder head, the adjustment tool must be mated/inserted without interfering with the surrounding parts inside the cylinder head, making this operation extremely difficult at times, or essentially impossible, hence requiring further improvements.